The PSI was founded on 4 October 1922 by the reformist wing of the
Italian Socialist Party (
Partito Socialista Italiano, PSI). The new party was led by
Giacomo Matteotti,
Vittorio Emanuele Modigliani,
Rinaldo Rigola,
Giacomo Treves, and
Filippo Turati after they had been expelled at the PSI party congress in October. A staunch opponent of
Benito Mussolini and
Italian fascism, Matteotti was assassinated by a
fascist secret police squad on 10 June 1924, an event that provoked the
Aventine Secession. Outlawed in November 1925, the PSU remained active as the clandestine Italian Workers' Socialist Party (
Partito Socialista dei Lavoratori Italiani, PSLI). On 19 July 1930, the PSLI re-joined the PSI. Leading members and activists of the party included
Anna Kuliscioff,
Oddino Morgari,
Sandro Pertini,
Camillo Prampolini,
Carlo Rosselli,
Giuseppe Saragat, and Treves. The party was a member of the
Labour and Socialist International between 1923 and 1930. The same PSU name was adopted in 1949 after Saragat and others left the PSI. In 1969, a new PSU was formed following the split from the unified PSI and Saragat's
Italian Democratic Socialist Party (
Partito Socialista Italiano Democratico, PSDI) and was favourable to the continuation of the
organic centre-left governing coalition. This group reverted to the PSDI name in 1971. == Electoral results ==